The production of magnetic recording structures, such as tapes and disks, is known. In these known processes, a liquid coating composition which has been pigmented with particles having the capacity to retain a magnetic orientation, referred to hereinafter as magnetizable particles, is applied to an appropriate nonmagnetizable substrate. The pigmented coating is then baked to solidify the coating and fix the magnetizable particles in place within the coating.
In the known processes, the magnetizable particles are used in admixture with a nonmagnetizable binder (which may be thermoplastic or thermosetting) and a volatile solvent is present to dissolve the binder and provide a solvent solution which is applied as a thin coating upon the substrate. The coated substrate is then force dried to remove the solvent. Some reactive binders are cured with polyisocyanates by prolonged storage in a hot room, e.g., at 140.degree. F. An illustrative process is shown in U.S. Pat. No. 4,246,316.
Unfortunately, the known processes are slow, and the cure used may lack desired uniformity. Also, when isocyanate-functional materials are used, these have a limited pot life and are hard to work with. It is desired to employ electron beam radiation to rapidly cure a polyethylenically unsaturated coating liquid on the nonmagnetizable substrate, but the cured coatings produced in this fashion lack the physical toughness needed to provide a good product. In this connection, reference is made to Japanese Application No. 48-126778 which was laid open June 24, 1975 under No. 50-77433.
It should be appreciated that the electron beam curing process does provide many advantages, such as the use of stable coatings, greater speed of production, more uniform product quality, more compact equipment, and, per this invention, products of greater durability in which the magnetic pigment is more easily and uniformly distributed.
In my prior application Ser. No. 528,044 filed Aug. 31, 1983, I disclosed coating compositions which can be pigmented with magnetizable particles and applied and cured with electron beam radiation to provide magnetic recording media. However, it has been difficult to get good pigment wetting without extensive pretreatment of the pigment or by modifying the coating resin in a manner which unduly softens or degrades the system, and this limits the ease and uniformity of pigment dispersal. This invention is addressed to the problem of providing improved radiation curable coating compositions which are hard and tough and which also have good pigment wetting properties.